1. Field of the Invention
The present invention relates to a flaw detector or discriminator and a flaw detection method for metal pipes and, more particularly, to a flaw detector or discriminator and a flaw detection method for maintaining pipelines such as underground gas pipelines, chemical plant pipelines and heat exchanger pipelines by means of remote field eddy current examination.
2. Description of the Related Art
Hitherto, a remote field eddy current sensor using a remote field eddy current examination method in detecting corrosion or the like existing in interior and exterior walls of a metal pipe such as an underground gas pipeline, a chemical pipeline or a heat exchanger pipeline is known in the art. As shown in FIG. 8, a remote field eddy current sensor 1 has an exciting coil 2 and one or more receiving coils 8 disposed in the longitudinal direction of the pipe and separated from the exciting coil 2 by a distance about twice the diameter of the pipe. The remote field eddy current sensor 1 is connected to a signal transmission cable 4 and, when it is inserted in a pipe 5, is allowed to run within the pipe 5 by means of a driving mechanism controlled from outside. The remote field eddy current sensor 1, together with an exciting signal generating means for applying an exciting signal to the exciting coil 2 of the remote field eddy current sensor 1 from outside the pipe via the signal transmission cable 4 and a flaw data generating means for creating flaw data by receiving measured signals from the receiving coils 3 of the remote field eddy current sensor 1 via the signal transmission cable 4, constitutes a flaw detector for metal pipes.
An exciting voltage from several volts to several tens of volts is applied to the exciting coil 2 of this remote field eddy current sensor 1 and an electromagnetic wave having a relatively low frequency normally from several tens of hertz to several hundreds of hertz is used as the exciting signal applied to the exciting coil 2. Electromagnetic waves caused by the remote field eddy current generated from the exciting coil 2 to which the exciting signal has been input propagate by indirect propagation passing through the thickness of the metal pipe or direct propagation in the pipeline as a wave guide. In the case of the latter however, the electromagnetic wave rapidly attenuates and is not substantially propagated because the frequency of the electromagnetic wave caused by the remote field eddy current is much lower than that of the pipeline. Conversely, in the case of the former, the electromagnetic wave caused by the remote field eddy current propagates through the thickness of the metal pipe while slowly attenuating and at the same time, part of it permeates into and again passes through the thickness of the metal pipe and is received by the receiving coil 3. Although the received signal detected by the receiving coil 3 is very weak (several .mu.V to several tens of .mu.V), the phase thereof is changed due to a skin effect in passing through the thickness of the metal pipe. Because this phase change has good linearity relative to the thickness of the metal pipe, a decrease of the thickness of the metal pipe may be accurately detected and the existence of corrosion and the depth of flaws on the interior and exterior walls of the metal pipe may be reliably detected by detecting the phase difference between the exciting signal and the measured signal.
However, the remote field eddy current sensor 1 described above has a drawback in that although the electromagnetic wave caused by the remote field eddy current and transmitted through the thickness of the undamaged metal pipe having no corrosion is transmitted mainly in parallel with the axis of the metal pipe and is favorably received by the receiving coil installed coaxially with the metal pipe, it cannot detect phase change accurately because an electromagnetic wave vertical to the interior wall of the pipe is generated at a defective portion and the receiving coil installed concentrically with the metal pipe cannot adequately receive this electromagnetic wave vertical to the longitudinal direction of the metal pipe.
Further, because the electromagnetic wave caused by the remote field eddy current transmitted through the thickness of the metal pipe is influenced by conditions such as the permeability and electrical conductivity of the material of the metal pipe and the frequency of the exciting signal, the phase difference of the measured signal output from the receiving coil disperses among pipes of different materials, even if an equal exciting voltage is applied to the exciting coil. Due thereto, the remote field eddy current sensor 1 has the problem that it cannot accurately evaluate the depth of a flaw in pipes having different materials from a predetermined relationship between the phase difference and the thickness.
The remote field eddy current sensor 1 also has the problem that it is difficult to detect a flaw by a single scan and scanning must be repeated many times when testing a pipe having a relatively large aperture (e.g. 200 A=e.g. outer diameter 225.8 mm, thickness 10.2 mm) with a remote field eddy current sensor in which one receiving coil is installed.
Accordingly, it is a primary object of the present invention to solve the aforementioned problems by providing a flaw sensor for metal pipes which can detect a magnetic field generated from a defective portion and can effectively test the entire interior circumferential surface of the metal pipe by a single scan using a plurality of receiving coils whose axes are disposed at right angles to the longitudinal direction of the metal pipe and which are provided in the circumferential direction of the pipe.
It is another object of the present invention to provide a method for detecting flaws in metal pipes which allows accurate evaluation of the existence of corrosion and the depth of the flaw in the metal pipe corresponding to the material of the metal pipe by detecting a phase difference which changes depending on the magnetic characteristics entailed by a difference in the material used to produce the metal pipe.